Under physiologic conditions, nitric oxide (NO) is exceedingly unstable, reacting essentially instantaneously with oxygen, superoxide anion, and redox metals (Lancaster et al., Proc. Natl. Acad. Sci. USA, 87:1223-1227 (1990); Ignarro et al., Circ. Res. 65:1-21 (1989); and Gryglewski et al., Nature 320:454-456 (1986)).
The consequences of NO production in the lung are not known. However, it has been believed that potential beneficial bronchodilation effects of NO may be counterbalanced by generation of toxic nitrogen oxides that form readily under the high ambient concentration of oxygen and other reactive oxygen species. Introduction of NO into the lungs has been associated by some with adverse effects, which occur as a direct result of the particular chemical reactivity of the uncharged NO radical (NO.sup..multidot.). These adverse effects create impediments to NO therapy which generally involves administration of NO.sup..multidot.. For example, the reaction between NO.sup..multidot., and O.sub.2 or reactive O.sub.2 species which are present in high concentrations in the lung, generates highly toxic products, such as NO.sub.2 and peroxynitrite. These reactions also result in the rapid inactivation of NO, thus allegedly eliminating any beneficial pharmacological effect. (Furchgott R. F. et al., I. Endothelium-Derived Relaxing Factors and Nitric Oxide; eds. Rubanyi G. M., pp. (1990); Gryglewski, R. J. et al., Nature 320:454-456 (1986)). Furthermore, NO.sup..multidot. reacts with the redox metal site on hemoglobin to form methemoglobin, which inhibits oxygen-hemoglobin binding, thereby significantly reducing the oxygen-carrying capacity of the blood.
Nonetheless, some workers have convincingly demonstrated the value of NO therapy in bronchoconstriction and reversible pulmonary vasoconstriction. For example, Zapol and Frostell, PCT Publication No. WO 92/10228 discloses a method for treating or preventing bronchoconstriction, e.g., asthma or reversible pulmonary vasoconstriction, e.g., pulmonary hypertension, by inhalation of gaseous nitric oxide or nitric oxide-releasing compounds. Many such compounds are known. These investigators characterize the mammalian circulatory system as consisting of two separate circuits, the systemic circuit and the pulmonary circuit which are controlled by opposite sides of the heart. They report that (since NO gas which enters the bloodstream is rapidly inactivated by combination with hemoglobin) the bronchodilatory effects of inhaled NO are limited to the ventilated bronchi and the vasodilatory effects of inhaled NO are limited to those blood vessels near the site of NO passage into the blood stream: i.e., pulmonary microvessels. They conclude from this that an important advantage of their bronchodilating and pulmonary vasodilating methods is that one can selectively prevent or treat bronchospasm and/or pulmonary hypertension without producing a concomitant lowering of the systemic blood pressure to potentially dangerous levels and that, therefore, their method allows for effective reversal of pulmonary hypertension without the risk of underperfusion of vital organs, venous pooling, ischemia, and heart failure that may accompany systemic vasodilation. More specifically, they report that the rapid binding of NO to hemoglobin ensures that any vasodilatory action of inhaled NO is solely a local or selective effect in the blood vessels of the lung, with no concomitant vasodilation downstream in the systemic circulation.